In the field of electronics, silicon carbide (SiC), gallium nitride (GaN) aluminum nitride (AlN), and other similar materials are used in crystal form. Applicants' invention includes an improved system and method for growth of these crystals. Applicants' invention will be described in part as it applies to SiC crystal growth, but it can be used to grow GaN, AlN, and other similar crystals.
The traditional method of growing SiC is by seeded sublimation growth. In this method, a graphite crucible is filled with SiC powder and a SiC single crystal seed is attached to the lid of the crucible, which is then sealed. The system is heated to temperatures above 2000° C. where SiC sublimes, transforming from solid to vapor.
More recently, SiC had been grown using gas-fed techniques, in which precursors are introduced into the reactor by flowing them into the reactor in the gas phase, instead of using powders as is done in seeded sublimation. Two gas-fed techniques are High Temperature Chemical Vapor Deposition (HTCVD) and Phase Controlled Sublimation (PCS).
A variation of the PCS technique is Hydride Vapor Phase Epitaxy (HVPE). In HVPE, silicon tetrachloride (tetra) is transported together with an argon (Ar) carrier in the outer tube of a coaxial injector. The Ar carrier helps to insulate the inner tube containing the flow of hydrocarbon, which is ethylene or methane. The hydrocarbon is transported in a hydrogen carrier. The gases mix in the hot zone and the tetra decomposes and SiC is deposited on the seed.
Gas-based processes for SiC growth take place in a reactor. The reactor typically includes a crucible having a lid containing a seed holder, which is used to hold a SiC crystal. The SiC crystal serves as a seed for growth of additional SiC, which is deposited onto the seed. The silicon and carbon needed to grow the SiC are provided through a flow of gaseous precursors, for example silane and ethylene, which enter the crucible through one or more feed lines that are connected to an injector.
These gas-based processes operate at high temperatures. High temperatures and reactive precursor gases create a harsh environment that leads to wear and degradation of reactor components. This wear requires frequent replacement of reactor components and also creates contaminants that reduce the quality of the SiC crystals.
Traditionally the crucibles, injectors, and other reactor components have been made from graphite. More recently, there has been interest in components made from graphite coated with metal carbides and metal nitrides. Metal carbides and metal nitrides have several advantages: they have very high melting points; they are hard and strong; they are electrically conducting; and they are chemically inert to most chemicals also at high temperatures. For these reason coatings of metal carbides on graphite, e.g. TaC and NbC coatings, have been used to extend the life of graphite parts in harsh environments.
The metal-carbide and metal-nitride coatings are difficult to apply and they typically fail due to a mismatch of the Coefficient of Thermal Expansion (CTE) between the graphite and the metal carbide. For applications where thermal cycling is normal, the failure of the coating will become frequent. Furthermore in conditions where deposits of other materials occur on top of the metal carbide coating, the failure frequency is further increased.
There has been significant work using tantalum metal crucibles for silicon carbide crystal growth applications. The inner Ta surface is converted to TaC. Although these crucibles are normally quite durable, the lattice mismatch between Ta and TaC is very large; and an enormous stress is built into the thin converted carbide layer.
In gas-fed systems for crystal growth, the injector is the most exposed part of the reactor. The high temperatures in combination with the severe chemical reactivity of the silicon vapor and hydrogen gas make it absolutely necessary to use innovative materials or coatings for the injector. With a bare uncoated graphite injector, the opening diameter of the injector will increase from 25 mm diameter to 45 mm diameter in only 10 hours due to silicon and hydrogen etching. This increase in diameter reduces the growth rate of the crystal and changes the growth profile. Furthermore, large amounts of loose graphite particles are carried away with the gas stream and embedded into the growing crystal.
In an early attempt to reduce injector wear, a TaC coated injector was used. The injector worked, but the coating only lasted for one run; and then a new injector was required.
Accordingly, there is a need for reactor components with enhanced resistance to the high temperatures and corrosive gases described above. Similarly, there is a need for methods of growing SiC and similar crystals to take advantage of the new reactor components. There also is a need for methods for improving the resistance of reactor components to this environment.
Finally, there is a need for crystals and electronic components taking advantage of the superior crystal properties that rise from use of the devices and methods of the invention.